1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus with a magnification changing device.
2. Description of the Prior Art
As the magnification changer of an image forming apparatus, there exist a device using a zoom lens and a device changing the length of a total optical path, the latter of which can establish a wider magnification change than the former at a lower cost so that it is adopted in a high-performance apparatus. An electrophotographic reproducing machine employed in this style will be described with reference to FIGS. 3, 4 and 5.
The machine is arranged with: a first carriage A as first running means adapted to run at a scanning speed while carrying a light source lamp 2 and a mirror 4 for irradiating a document on a document glass plate 80; a guide rail (not shown) for said first carriage A; a second carriage B as second running means movable with running blocks 12 and adapted to run in the same direction as the first one A while carrying mirrors 5 and 6 (which are generally called collectively a "V-mirror"); a guide rail (not shown) for said second carriage; a block carriage 70 made movable in the same direction as the first carriage for changing the position of the V-mirror to change the magnification; a lens carriage 40 carrying a projection lens 7; a mirror 8; and a photosensitive drum 9 for providing a focal plane.
When the lens carriage 40 and the block carriage 70 are moved, the V-mirror 5 and 6, i.e., the initial position of the second carriage B and the lens 7 are set in the respective positions for non-enlargement, reduction and enlargement of sizes, as shown in FIG. 3.
Turning to FIG. 5, a wire is made to run on each of pulleys 11, which in turn are driven by a motor 10. The wire has its front half W.sub.1 run on the running block 12 and, through an intermediate pulley, on a pulley 14 carried on the block carriage 70 until it is fixed to said carriage. On the other hand, the rear half W.sub.2 of the wire is made to run through several pulleys on the aforementioned running block 12 in the direction opposite to the front half W.sub.1. The rear wire half W.sub.2 is further made to run, through an intermediate pulley, on a pulley 22 carried on the block carriage 70 until it is fixed on said carriage 70.
The power train described above is symmetrically provided also at the other side, as shown in FIG. 5.
In accordance with each magnification desired, on the other hand, the lens 7 on its carriage 40 is moved to and set in specific positions, as will be described with reference to FIG. 4. The rear wire half is made to run on a pulley 28 driven by a stepping motor 58 and is fixed through a pulley 32 on the lens carriage 40. The front wire half is made to run on pulleys 29, 30 and 31 and is likewise fixed on the lens carriage 40. A cam 85 is pivotted by the shaft of the pulley 30 so that it can rotate together with the pulley 30. As a result, the stepping motor is energized in accordance with a desired magnification to run the wire thereby to determine lens positions and corresponding cam angular positions. A cam follower 86 carried on the block carriage 70 is then pushed by the cam 85 to determine the initial position of the block carriage so that the initial position of the V-mirror is set in accordance with the lens position of each magnification, thus changing the magnification.
The movement of the block carriage 70 is doubled with respect to that of the initial position of the second carriage B, i.e., the V-mirror for changing a magnification. This movement increases with increasing change in the magnification. Specifically, the difference of the initial position of the V-mirror is about 60 mm between the magnification M=0.5 and M=1, and the movement of the block carriage 70 is doubled to about 120 mm. This means the large movement of the block carriage and the stroke of the cam 85. This accordingly enlarges the space to be occupied by those members, thus seriously obstructing the size reduction of the apparatus. This obstruction becomes more serious with larger change in the magnification.